This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We are examining the structure of nitric oxide synthase (NOS) to understand the conformational changes inherent in its activation by calmodulin (CaM). NOS is the enzyme that generates the intracellular NO signal that influences physiological processes in myraid cell types;biomedically, its role in the constriction of blood vessels is critical in regulating blood pressure. We have imaged and reconstructed the endothelial form of the enzyme (eNOS), in the absence and presence of Ca++/CaM, by cryo-electron microscopy using our local electron microscope facility, a JEOL 1200EXII. The structural resolution of each reconstruction is estimated as 20-25A. we propose to extend the resolution by using the NCMI facility.